Comments

The following patch stops the segfault (which happens because
cpu_unlink_tb() is fiddling with the links between tbs without taking
the tb_lock, so another thread can come in via eg tb_add_jump() and
cause corruption of the linked lists). However, there are a number of
comments in the TB handling code about things being non-thread-safe or
not SMP safe, so I need to have a more careful think about the whole
thing.